Method and Apparatus For Processing a Dry Fuel

ABSTRACT

The invention relates to a method and device for processing a dry fuel, in particular a wood pallet, wherein a dust-binding additive is added to the conveying medium that transports the dry fuel. The conveying medium that transports the dry fuel comprises an air flow fed from a high-pressure or low-pressure source, the dust-binding additive being added to the air flow in the form of a mist.

PRIORITY CLAIM

This application is a United States National Stage Application claiming the benefit of priority under 35 U.S.C. 371 from International Patent Application No. PCT/DE2010/001306 filed Nov. 9, 2010, which claims the benefit of priority from German Patent Application Serial No. DE 102009052643.9 filed Nov. 10, 2009, the entire contents of which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for processing a dry fuel, in particular a wood pellet, such that the dry fuel is transported over a predetermined transport route with the aid of a conveying means, characterized in that an additive which increases elasticity, lowers material stresses and at the same time lubricates and binds dust is added to the to the dry fuel on the transport route through the intermediary of the conveying means, and wherein a dry fuel, in particular a wood pellet, characterized in that the dry fuel is coated at its surface with an additive such that the surface of the dry fuel presents an increased elasticity, reduced internal stresses, and a property of binding dust due to adhesion.

BACKGROUND OF THE INVENTION

Large quantities of sawing by-products are generated in the wood-processing industry. Thus, no more than approximately 60% of a log's volume can be converted into a main product, with the remaining 40% having to be termed waste in the widest sense. The latter primarily include sawdust, wood shavings, and chips of which wood pellets are usually produced. By way of example, such a rod-shaped wood pellet has a diameter of between 4 mm and 10 mm, a length of between 5 mm and 50 mm, a bulk density of about 1.1 to 1.3 kg/dm³, and an energy density of about 5 kWh/kg. The manufacture of the pellets takes place in a ring die or flat die pelleting press that is known per se, wherein crushing rollers press the material to be pressed through the bores of the die. One essential precondition for high-grade pellets is a starting material having a highest possible fineness. Thus, the dried wood particles are finely ground with the aid of fast-moving hammer mills before entering the presses. Inside the presses the wood particles are then ground even more finely in a combined grinding and pressing process by the rotary movements of roller and die through the application of high shear forces, and are pushed through the bores in the die. This engenders temperatures of more than 120° C. and very high pressures. The pressures are highest on the walls of the bores. In comparison with feedstuff pellets, for instance, the specific energy input for wood pellets is higher by a factor of 3-5.

As a consequence the cylindrical surface of the pellets presents an extremely high density and a smooth surface. The boundary area accordingly is substantially hard but brittle. Following their passage through the bores the pellets are cut, then drop into the cooler where the pellets lose another 2%-3% of their humidity by releasing water vapor to the air passing through, are cooled further, and undergo surface hardening. In the storage silo the core temperature of the pressed pieces remains elevated for another 1-3 weeks in comparison with the surface temperature. These factors jointly result in several properties that are specific for wood pellets. Thus, the finished pellets are for a large part made up of extremely fine wood flour. An average breakdown of 6-mm pellets by fractions might be as follows:

<3.15 mm 98.00% <2.0 mm 95.00% <1 mm 63.00% <0.5 mm 25.00% These figures illustrate the fineness at which the particle sizes are present within the pellets.

Wood pellets for heating purposes have a residual humidity of about 6% to about 10% at the most. This humidity content, which is very low for wood, is necessary in heating pellets in order to ensure a high energy density in combination with a good ignition behavior. The humidity content at the cylindrical surfaces is lower than in the core areas. As a result, the cylindrical surface shrinks and due to the extremely low water content becomes very brittle and fragile, and is subjected to tensile stresses. The starch as a pressing aid moreover forms a so-called pressing skin on the cylindrical surface of the pellets, which is equally fragile and brittle because of the high dry content. Due to the inhomogeneous pressure distribution inside the bore of the die, the apparent density is distributed non-uniformly over the cross-section of the pellets. Starting from the center, the density increases radially toward the cylindrical surface. These factors in combination with the high dust content result in an inherent tendency of wood pellets to generate relevant quantities of abraded matter in the form of fragments and dust during every manipulation.

Wood pellets for heating purposes are delivered to the end consumer in two ways: On the one hand in the form of bagged product on pallets, mostly for use in pellet stoves; on the other hand, in the form of loose product for use in central heating in the usual performance range between 10 kW and 1,000 kW. The loose product is customarily delivered to the customer by means of silo trucks and injected via tube conduits with the aid of a pressure system (reservoir +compressor). This technique has been known and accepted for decades. Silo reservoir trucks are used to transport, e.g., cereal flours, animal feeds and granulates of all kinds and inject them into the provided storage containers. In these known applications the product to be conveyed either already is in dust form (flour), or on the other hand the tendency to form dust is substantially less pronounced than in the case of wood pellets (example: feedstuffs that are substantially more elastic because of their oil/lipid content, that are less highly compacted, do not present material stresses like wood pellets, contain less particulate matter, and already possess properties of binding fines contents). In the case of wood pellets the excess air is evacuated from the pellet storage to be filled (bag silo, storage space) via an extractor fan. The storage space is accordingly provided with two ports mostly having a size of 100 mm, namely, one for injection and one for evacuation. Injection distances of 20 meters are common, with up to 50 meters being technically viable. Injection heights of up to 30 meters can be overcome in a vertical direction. During injection the wood pellets are subjected to high mechanical strains which depend on the tube lengths and local circumstances. Owing to the material properties of wood pellets described in the foregoing, this results in the generation of so-called abraded matter having the form of wood dust and fragments. In addition, a certain quantity of dust inherently clings to the wood pellets, which is then set free by the movement and the air flow during injection. Downstream from the extractor fan a filter sack is provided as a preventive measure against the high dust load. Within the storage a large part of the generated fine dust does, however, settle on and in the voids between the pellets, especially in zones where the air flow may be calmed or circulates. This is where so-called “nests of dust” may additionally form, i.e., areas having an elevated proportion of fines. So-called fragments are furthermore produced inside the injection conduit. These are compacted grains having sizes of about 0.1 mm to 3.15 mm (under the DIN standard [German Industrial Standard], particles>3.15 mm are not termed fines any more). A high proportion of dust and fragments degrades the flow characteristics of the pellets in the storage and in the conveying means leading to the pellet heating system proper, may cause clogging within worm conveyors, brings about an unpleasant fine dust load in the basement and housing space in the case of the widely common vacuum systems, degrades the ignition behavior in the burner, with the possibility of the fine dust proportion also increasing in the flue gases in consequence, combustion in general is degraded (degraded air management in the primary air area), and altogether brings about a degradation of the boiler efficiency.

DE 39 18 523 C2 proposes a method for preventing dust formation during loading or transport of fertilizer granules, wherein a liquid mixture of molasses and glycerol or polyethylene glycol or triethanolamine as a second organic substance are added as a dust binder. Such additives are not acceptable in the case of a fuel because of the toxic effect of these substances during combustion.

SUMMARY OF THE INVENTION

The present invention is based on the object of providing an environmentally compatible and at the same time low-cost solution for processing a dry fuel, in particular a wood pellet, which renders the dry fuels more resistant to mechanical stresses. This objective is achieved through the method and apparatus as well as the dry fuel of the present invention.

The method of the invention and the apparatus of the invention for processing a dry fuel, in particular a wood pellet, are characterized in that an additive which increases elasticity, lowers material stresses and at the same time lubricates and binds dusts is added to the dry fuel on the transport route by means of the conveying means. Here it is provided in accordance with the principle of the invention that the conveying medium transporting the dry fuel includes an air flow fed by a high-pressure or low-pressure source, and the dust-binding additive is added to the air flow in the form of a spray mist. Advantageously it is provided that the addition of the dust-binding additive takes place immediately after the entry of the dry fuel into a delivery conduit. In accordance with the invention the addition of about 1 to 3 liters of additive is required for every 1,000 kg of dry fuel.

The advantage of the invention resides in the fact that the surface of the dry fuel is given dust-binding properties while at the same time the flow characterristics and thus the transportability of the fuels are improved considerably, the material stresses are lowered, and the surface of the fuels is rendered more elastic.

An advantageous development of the invention consists in the addition inside the injection conduit of a spray mist which collides with the wood pellets and encloses them with a thin oil film or emulsion layer. Moreover the dust particles may collide with droplets of the additive and furthermore bind to larger-sized wood pellets.

One important effect in the framework of the invention resides in the fact that lowering the surface stresses and raising the surface elasticity globally causes less abraded matter to be produced during manipulation of the pellets. Apart from practical tests involving a silo injection truck, this effect could also be proven through the so-called abrasion test. The abrasion test describes a standardized method for testing the mechanical abrasion resistance of wood pellets that is widely common in the pellet industry and prescribed by DIN. In this test, 100 g of pellets are whirled about inside a defined air flow during 1 minute, and afterwards the produced fines content is reweighed. Abraded matter values of 1-2% are standard. By means of the surface treatment of the pellets it was possible to lower the quantity of abraded matter.

A second effect consists in the removal of dust due to the fact that the pellets are coated with a tacky, at least dust-binding layer by spraying in the additive, so that the dust particles may bind to this surface. Another effect of rather minor importance is that small dust particles combine with each other to form larger-sized particles as they collide with each other. The coating prevents the dust adhering to the pellets from detaching. Thus it is crucial that the surface of the pellets is covered by a dust-binding layer.

It is one positive side effect of the utilization of oils, waxes or resins that automatic ignition by the burner is facilitated.

In principle any sprayable substance that reduces material stresses, increases elasticity, possesses dust-binding properties, does not or only insubstantially increase the calorific value, and does not bring about unfavorable properties during combustion, may serve as the spraying liquid. Ideally, however, the additive should have the following properties:

-   -   1. Reduce the material stresses and/or increase the surface         elasticity     -   2. Lubricate the inner surfaces of injection conduits, worm         coils and conveying spirals     -   3. Possess dust-binding properties     -   4. Present a low surface tension     -   5. Have no properties degrading combustion, or generate noxious         exhaust gases     -   6. Be low-cost, and     -   7. Be of natural origin

Advantageously it is provided that the coating additive includes an oil and/or a molten or liquid wax and/or a lipid. Some oils possess the best properties. Before the background that the standard heating systems are adapted to the energy content of wood having a humidity content of about 6-10%, however, it is crucial to consider a possible undesirable augmentation of the calorific value. The burners of pellet furnaces and boilers have a certain control range which is, however, rather restricted due to their construction. The feed augers for conveying the pellets into the firing chamber are as a general rule controlled and preset volumetrically. In other words, if the calorific value of the pellets per volume unit is significantly (more than about 3%) higher than that of untreated wood pellets, there is a possibility of excessive energy being supplied to the burner. As a consequence the heating system then is frequently not capable any more of sufficient readjustment control, with a deficiency of air or oxygen being created inside the firing chamber. This causes the combustion to become less clean, and more exhaust gases are produced, involving the risk of the formation of ashes. In this regard, care should be taken to keep the added amount of an additive increasing the calorific value as low as possible or necessary. In trials including canola oil, e.g., 1 to 3 liters per metric ton (corresponding to approx. 0.1-0.3%) were already sufficient in the form of a spray mist application inside the injection conduit in order to attain the desired advantages. In this case the calculated increase of the calorific value is only 0.2% to 0.6% per volume unit. The required quantity specifically depends on the quality of the wood pellets. Pellets of a poorer quality which present—or create during transport—a higher content of abraded matter and dust require more additive.

As the wood pellets are an ecological fuel, however, preference should at any rate be given to natural substances. In this sense it is advantageous if the coating additive includes a mixture of water with swollen starch and/or gelatin and/or glue. The effect is improved by lowering the surface tension with the aid of a tenside. In practice a water-based additive is not expected to be employed as it will cause the pellets to swell even in cases of slight excess quantities or droplet formation. We could not observe such easy and rapid swelling when oils, waxes or resins were used.

An additional advantage is brought about if the spraying device is installed in a location of the injection conduit downstream from where the pellets enter from the reservoir into the delivery conduit. As a result, the spray mist is already mixed homogeneously with the injection air prior to the actual contact with the pellets.

The efficiency of the dust binding process is enhanced by the fact that the velocity of flow of the air inside the injection tubes is substantially (by about a factor 10) higher than the flow velocity of the pellets (pellets=approx. 5 m/sec, air>50 m/sec). In addition the air has to meander, as it were, through the pore passages of the pellets while undergoing numerous and lasting changes of direction and velocity. Photographs of pellets flowing inside the injection conduit showed that the pellets have a relatively uniform distribution and are afloat separately in the air flow. As a result there is a very high and uniform probability of the pellets or particles to collide with the droplets of spray mist. Furthermore a thin oil film is deposited on the walls of the injection conduits. This thin oil film in addition allows the pellets to slide more smoothly on the walls, and dust and fragments may also adhere there before again being carried along and bound by pellets. All in all this has a very strong effect of reducing the fine dust content.

Spray liquids found to be suitable were, for example:

vegetable oils, canola oil, linseed oil, sunflower oil

mineral oils such as, e.g., glycerol, paraffin oils,

water-based additives on the basis of swollen starch or gelatin

wax emulsions and lipids

An additional advantage is that the compressed air in the injection conduits is heated. As a result it is also possible to use substances that are normally semi-solid or solid but not brittle at ambient temperature. These do, however, have to be pre-heated in a storage container (lipids, waxes, sterols, etc.). The further advantages of the processing of dry fuels in accordance with the invention by means of the present method are as follows: The frictional resistance of the pellets as a bulk material is reduced, with the flowability of the pellets in the storage space/silo consequently being improved. The oily coating lubricates the injection conduits and the conveying means leading to the heating burners (screws, spirals, suction conduits). According to our observations, the oils do not soak completely into the inside of the pellets but a lasting thin “lubricating film” is preserved (hardly any cavities present in the extremely compacted pressed pieces). The slight increase in pellet stability reduces the generation of abraded matter even following injection of the pellets into the end consumer storage from which the pellets must then again be conveyed one last time—sometimes via several stations—to the burner.

The apparatus of the invention for transporting a dry fuel, in particular a wood pellet, comprising a conveying means which transports the dry fuel with the aid of a conveying medium, in particular an air flow fed by a high-pressure or low-pressure source, is characterized in that the conveying means is associated with a feed means whereby an additive which increases elasticity, lowers material stresses and at the same time binds dusts is added to the conveying medium transporting the dry fuel. Here it is provided that the conveying medium transporting the dry fuel includes an air flow fed by a high-pressure or low-pressure source, and the additive is added to the air flow in the form of a spray mist.

In accordance with a quite particularly preferred development of the invention it is provided that the additive has an evaporation temperature that is lower than the ignition temperature of the dry fuel. It is advantageous if the additive, such as a vegetable oil, is crosslinked during heating in the burner to thus form a viscous resinous coat. As a result the so-called aerosols (fine dusts) are also retained in the coat, and especially the organic constituents undergo postcombustion. Aerosols are predominantly formed during the pyrolysis phase in which the combustion process is not clean yet. This is precisely why it is advantageous if the vaporization point of the additive is lower than the ignition point of the fuel. In this way, the coating ignites before it burns up, with the aerosols undergoing postcombustion as a result. In addition an early, superficial back-up fire is created which makes combustion cleaner. Here it is necessary to consider each individual pellet. The life cycle of a pellet lasts about 3 minutes and is divided into 4 phases:

1) the evaporation phase (water vapor escapes, <100° C.)

-   2) the pyrolysis phase (heating on the bed of glowing embers, <about     350° C.) -   3) the gas phase (with flame) -   4) the ember phase.

The additive coating has the effect of the pyrolysis phase being shortened to some extent and the clean gas phase commencing somewhat earlier. Thus, in the case of canola oil the ignition point is at about 320° C. but appears to even be raised, just like the vaporization point, due to crosslinking of the additive coat. The precise chemical processes have not been examined sufficiently yet.

In a preferred development of the invention it may be provided that the additive is taken to a predetermined temperature prior to its application on the dry fuel. Here it is important to pre-heat the oil to a fixed and adjustable temperature prior to spraying on the pellets, for the viscosity changes greatly with the temperature. This temperature is at about 35° C. to allow good atomization of the oil, at pressures of about 10 bars to 20 bars.

In an advantageous development of the apparatus of the invention it is provided that the addition of the dust-binding additive takes place immediately following the entry of the dry fuel into a delivery conduit.

In a particularly preferred development, the apparatus of the invention represents a utility vehicle for use on a road or waterway.

The invention further relates to a dry fuel for heating purposes, in particular a wood pellet, which is characterized by the fact that the dry fuel is coated at its surface with an additive such that the surface of the dry fuel presents an increased elasticity, reduced internal stresses, and a property of binding dust due to adhesion.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, advantages, expedient points and developments of the invention may be taken from the following description of a practical example making reference to the figures of the drawings, wherein:

FIG. 1 is a schematic sectional view of a practical example of the invention; and

FIG. 2 is a schematic sectional view of a further practical example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus 1 of the invention represented in FIGS. 1 and 2 for the transport and/or storage of a dry fuel 2, in particular a wood pellet, comprises a conveying means 3 which transports the dry fuel 2 with the aid of a conveying medium, in particular an air flow (arrow 4) fed by a high-pressure or low-pressure source (not shown in detail in the figures) and conducted inside an injection conduit 5, wherein the conveying means 3 is associated with a feed means 6 whereby a coating additive 7 is added to the conveying medium 4 transporting the dry fuel 2; namely, the additive 7 is added to the air flow 4 in the form of a spray mist. The feed means 6 comprises an additive reservoir 8 in which a predetermined pressure prevails and which on the output side feeds an atomizer nozzle 10 arranged in the injection conduit via an additive feeding conduit 9. As regards quantity and droplet size of the spray mist 7 emitted by the atomizer nozzle 10, the spray mist 7 is adjusted as a function of the transport route.

The dry fuels 2 stored in the pellet chamber 12 “flow” via a funnel-shaped chamber outlet 13 connected to the bottom side of the pellet chamber into the injection conduit 5 (in accordance with arrow 14) and are in a given case transported to a pellet storage (equally not represented) of a residential building via further tube conduits (not represented in detail) in accordance with arrow 11. The addition of the additive 7 takes place immediately following the entry 13 of the dry fuel into the delivery conduit 5. The injection conduit 5 and all of the conduits and tubes connected thereto are coated with the additive. This coating 15 has the effect of increasing strength, binding dust, lowering the sliding resistance of the dry fuels, and brings about a reduction of fragments and abraded matter. Inside the injection conduit 5 the dust particles bind to the dry fuel 2 and to larger-sized particles. The dry fuel 2 is coated at its surface with the coating 15 of dust-binding additive 7 such that the surface of the dry fuel 2 is given higher elasticity, abrasion resistance, slidability, and a property of binding dust through adhesion.

In FIG. 1 the basic effective principle of the invention is illustrated in a schematic sectional view.

FIG. 2 shows a preferred practical example of the apparatus 1 of the invention having the form of a silo truck including three pellet chambers 12 a, 12 b, 12 c, wherein selecting a pellet chamber may be affected with the aid of a slide valve 16 in order to open the desired pellet chamber 12 a, 12 b, or 12 c. 

1.-14. (canceled)
 15. A method for processing a dry fuel wherein the dry fuel is transported over a predetermined transport route with the aid of a conveying means, characterized in that an additive which increases elasticity, lowers material stresses and at the same time lubricates and binds dust is added to the dry fuel on the transport route through the intermediary of the conveying means.
 16. The method according to claim 15, wherein the dry fuel is a wood pellet.
 17. The method according to claim 15, wherein the conveying means comprises a conveying medium transporting the dry fuel, in particular an air flow fed by a high-pressure or low-pressure source, and the additive is added to the conveying medium or air flow in the form of a spray mist.
 18. The method according to claim 15, wherein the addition of the additive takes place immediately after the entry of the dry fuel into a delivery conduit.
 19. The method according to claim 15, wherein the additive is taken to a predetermined temperature prior to its application to the dry fuel.
 20. The method according to claim 15, wherein the additive has an evaporation temperature that is lower than the ignition temperature of the dry fuel.
 21. The method according to claim 15, wherein the additive is added to the dry fuel inside the injection conduit of a vehicle upstream or downstream from the feed conduit.
 22. The method according to claim 21, wherein the vehicle is selected from the group comprising a road silo pump car vehicle, a rail silo pump car vehicle, or a transport ship.
 23. An apparatus for processing a dry fuel comprising conveying means for transporting the dry fuel over a predetermined transport route, wherein the conveying means is associated with feed means whereby an additive which increases elasticity lowers material stresses and at the same time binds dust is added to the conveying means transporting the dry fuel.
 24. The apparatus of claim 23, wherein the dry fuel is a wood pellet.
 25. The apparatus of claim 23, wherein the conveying means comprises a conveying medium transporting the dry fuel, wherein the conveying medium is an air flow fed by a high-pressure or low-pressure source, and the additive is added to the conveying medium in the form of a spray mist.
 26. The apparatus of claim 23, wherein the addition of the dust-binding additive takes place immediately after the entry of the dry fuel into a delivery conduit.
 27. The apparatus of claim 23, wherein the additive is introduced at several different locations on the transport route.
 28. The apparatus of claim 23, wherein the temperature control means is provided whereby the additive is taken to a predetermined temperature prior to the application of the dry fuel.
 29. The apparatus of claim 23, wherein the additive has an evaporation temperature that is lower than the ignition temperature of the dry fuel.
 30. A dry fuel wherein the dry fuel is coated at its surface with an additive such that the surface of the dry fuel presents an increased elasticity, reduced internal stresses, and a property of binding dust due to adhesion.
 31. The dry fuel of claim 30, wherein the dry fuel is a wood pellet.
 32. The dry fuel of claim 30, wherein the dust-binding additive includes at least one component selected from the group consisting of an oil, a molten wax, a liquid wax or a lipid.
 33. The dry fuel of claim 30, wherein the dust-binding additive includes a mixture of water and at least one component selected from the group consisting of swollen starch, gelatin or glue. 